Cement kiln exhaust gases vary significantly and typically contain volatile heavy metals, such as mercury, which are generally volatilized from the raw materials and fuels during the clinkering process, and carried into the atmosphere. Cement kiln exhaust gases typically contain oxides of carbon, sulfur, nitrogen, alkalis, excess chlorides and volatile heavy metals such as mercury. Mercury in both its elemental and ionic form are generally continually emitted through the cement kiln exhaust stack in varying concentrations dependent upon the operation of the kiln, in-line raw mill and raw material or fuel inputs.
These gases may also be re-used for drying and heating within the inline raw mill and then exit the process as cement kiln exhaust gas. The heavy metals within the gases may then be released to the atmosphere after they pass through a kiln baghouse electrostatic precipitator, or other particulate collection apparatus.
Typical mercury concentrations in cement kiln exhaust gases may vary significantly and are highly dependent on the raw materials, process conditions, and fuels burned in the clinkering process at each site. Previous attempts to capture and contain mercury from cement kiln exhaust gas in both its elemental and its oxide form have generally had mixed results. These processes may also be expensive. These processes include activated carbon injection, flue gas desulphurization scrubbers, and sorbent technology. These processes do not generally render the residual mercury in a stable, non-leachable form which can be used as a process addition while having no detrimental effects on the resulting portland cement or concrete.
Treatment processes for power plants such as those disclosed in Hurley U.S. Pat Nos. 7,407,602, 7,771,683, and 7,776,294 are likewise inapplicable or inappropriate to the environment of the cement kiln for a variety of reasons.